Thermodynamic Conditions for the Nernstian Response of the Flat Band Potential of the Metal Oxide Semiconductor: A Theoretical Study
Jieqiong Li, Lingyi Meng, Jun Cheng
Abstract
The flat band potential (Ufb) is a fundamental property of the semiconductor/electrolyte interface and is dependent on the pH of electrolyte, as described by the (non-)Nernstian relation. However, the origin of (non-)Nernstian dependence between the pH and flat band potential of the metal oxide semiconductor is still in dispute. Herein, we extend a continuum double-layer (CDL) model to elucidate the thermodynamic condition of Nernstian behavior. By integrating the analytical derivation and CDL simulations, we confirm that when the reaction free energies are equal for the proton adsorption and desorption at the TiO2/electrolyte interface, the pH dependence of the flat band potential follows the Nernstian relation. Specifically, we verify this proposal by coupling the CDL model with the well-known Langmuir and Frumkin adsorption isotherms, with and without considering the lateral interactions between surface species, respectively. We demonstrate well that the thermodynamic balance between the proton adsorption and desorption processes determines the Nernstian response of the flat band potential.